1. Field of the Invention
The present invention relates to a control circuit of power converter, and particularly to a converter control circuit which controls the d.c. output voltage to be constant and the input current to be in phase with the power voltage.
2. Description of the Prior Art
FIG. 1 shows in block diagram a conventional power converter control circuit disclosed in an article entitled "Review of Control Characteristics of PWM Converters", in the proceeding of the 1985 national convention of The Institute of Electrical Engineers of Japan. In the figure, indicated by 1 is a power converter, which is a PWM converter in this example. Indicated by 2 is an a.c. filter reactor provided at the a.c. input of the converter 1, 3 is an a.c. power source which supplies an input current I.sub.S, 4 is a d.c. filter capacitor provided at the d.c. output of the converter 1, 5 is a load, 6a is a power voltage V.sub.S detecting circuit, 6b is a d.c. output voltage V.sub.D detecting circuit, 6c is an input current I.sub.S detecting circuit, and 6d is a load current I.sub.L detecting circuit.
Indicated in FIG. 1 are sections of a control circuit, in which are included a reference voltage generating circuit 101 for producing a reference voltage V.sub.DR' a subtracter 102 which calculates the difference between the detected voltage V.sub.D provided by the detecting circuit 6b and the reference voltage V.sub.DR to evaluate the voltage difference, a voltage control circuit 103 which produces a voltage control signal depending on the voltage difference, a feed-forward control circuit 107a which produces a feed-forward signal derived from the detected value I.sub.L from the detecting circuit 6d multiplied by a constant K.sub.L, an adder 107 which calculates the difference between the voltage control signal and the feed-forward signal to produce an input current peak command I.sub.m, a sinusoidal wave generating circuit 108 which produces a sinusoidal waveform sin .theta. in phase with the power source voltage V.sub.S based on the detected value V.sub.S of the detecting circuit 6a, a multiplier 109 which multiplies the peak command I.sub.m and the sinusoidal waveform sin .theta. to produce an input current command I.sub.SS, a subtracter 111 which calculates the difference between the detected value I.sub.S from the detecting circuit 6c and the input current command I.sub.SS to evaluate the current difference, a current control circuit 112 for producing a current control signal depending on the current difference, an adder 113 which adds the detected value V.sub.S of the detecting circuit 6a to the current control signal to compensate the disturbance of the power voltage V.sub.S, a carrier wave generating circuit 115 which produces a carrier signal, e.g., a triangular wave, a PWM (pulse-width modulation) circuit 114 which compares the output of adder 113 with the carrier wave to time the switching operation of switching devices (not shown) constituting the converter 1, and drive circuit 116 which activates the converter 1 depending on the output pulse width provided by the PWM circuit 114.
Next, the operation of the above converter system will be described. The converter 1 converts a.c. input power into d.c. power and supplies it to the load 5. The capacitor 4 is provided for absorbing the variation in the d.c. output voltage V.sub.D of the converter 1. The control circuit controls the d.c. output voltage V.sub.D so that it is equal to the reference voltage V.sub.DR, and also causes the input current I.sub.S to be sinusoidal in phase with the power voltage V.sub.S so that the system operates at a 100% power factor, with less harmonics and lower distortion factor.
In order to maintain a constant d.c. output voltage V.sub.D, the voltage control circuit 103 provides a voltage control signal for modifying the peak value of the input current I.sub.S. If the voltage control has a laggard response, an abrupt fall of the d.c. output voltage V.sub.D across the capacitor 4 could cause a control disability, and this problem is overcome by adding a feed-forward signal with a value of K.sub.L I.sub.L to the voltage control signal on the adder 107 so that the peak value command I.sub.m is instantaneously responsive to a load variation.
The input current command I.sub.SS is produced through the multiplication of the peak value command I.sub.m and the sinusoidal waveform sin .theta. in phase with the power voltage V.sub.S on the multiplier 109. The input current command I.sub.SS is subtracted by the input current I.sub.S on the subtracter 111 to evaluate the current difference, which is followed by the current control circuit 112 to produce the current control signal. The current control signal is added by the power voltage V.sub.S on the adder 113 so that the disturbance by the power voltage V.sub.S is compensated, and then the resulting signal is fed to the PWM circuit 114. The PWM circuit 114 compares the current control signal with a carrier wave, e.g., a triangular wave at 1-2 kHz, provided by the carrier generating circuit 115 to produce a PWM signal with a pulse width dependent on the values of voltage difference and current difference. The PWM signal is fed to the drive circuit 116, which operates the switching devices of the converter 1 accordingly.
The conventional power converter control circuit arranged as described above is intended to be responsive to an abrupt fall of the d.c. output voltage V.sub.D through the addition of the load current I.sub.L signal on a feed-forward basis to the voltage control signal for producing the input current command I.sub.SS. Consequently, in case of a single-phase inverter for the load 5, the load current I.sub.L has a significant amount of ripple, which appears in the input current command I.sub.SS' resulting disadvantageously in an increased harmonics included in the input current I.sub.S waveform.